Apparatus for plasma treatment of small diameter tubes

ABSTRACT

An apparatus for generating a plasma in the lumen of a small diameter tube includes a housing (12) having a diaphragm (18) which separates the housing into a first chamber (14) equipped with a gas inbleed (26) assembly and a second chamber (16) connected to a vacuum source. Parallel plate electrodes (58) in the first chamber are encased in a dielectric (42) which prevents substantially all plasma discharge external of a plasma zone between the electrodes. A conduit (64) maintains gas flow between the chambers. A bore (52) through the dielectric includes the plasma zone and receives the tube (54) to be plasma treated. A method for treating the luminal wall of a plastic tube (54) includes positioning a tube in the plasma zone, evacuating the chambers, bleeding a gas into the chambers, and delivering radiofrequency power to the electrodes (58). A long tube may be drawn through the plasma zone, or the dielectric and electrodes may be moved past the tube.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to techniques and implements to facilitate theattachment of cells to a surface, and more specifically relates to anapparatus for plasma treating the lumen wall of a small diameter tube inpreparation for cell deposition.

2. Background

Over the past three decades, vascular grafts have been used extensivelyto restore blood flow to areas of ischemia, to provide blood flow forhemodialysis patients and for repair of arterial aneurisyms. Suchprocedures are generally initially successful, but long term prognosisfor patients receiving small diameter grafts is not encouraging,principally because grafts of 4 mm or less become occluded over time dueto the thrombogenic nature of the graft material.

Extensive investigations have been carried out in attempts to find bloodcompatible materials for vascular grafts and other biomedical devices.Synthetic plastics are the preferred materials, but even such plasticsas polytetrafluoroethylene and the silicone rubbers which are morecompatible with blood than most plastics still show thrombogeniccharacteristics.

The problems of thrombogenicity and occlusion are exacerbated with smalldiameter grafts. Van Wachem et al., in Biomaterials 6, 403 (1985)reported clinical success with polymeric grafts of greater than 4 mm,but that grafts of less than 4 mm gave generally disappointing clinicalresults due to immediate occlusion. Likewise, Baker et al., in AmericanJournal of Surgery 150, 197 (1985) stated that long term patency oflarge diameter vascular grafts is relatively acceptable, but smalldiameter (less than 5 mm) grafts exhibit poor long-term patency rates.

The ideal blood-surface interface has long been considered to be thenaturally occurring human endothelium, and much current research centerson endothelialization procedures. For example, seeding of 4 mm IDdiameter polyester vascular grafts with endothelial cells and patencyafter implantation in dogs is discussed by Belden et al., in Trans. Am.Soc. Artif. Intern. Organs. 28, 173 (1982).

Jarrell et al. (Annals of Surgery, 203, 671 (1986)) disclosed a highpercentage of firm adherence of endothelial cells to polyester coatedwith platelet-rich-plasma in 10 min., to amnion/collagen coatedpolyester in 30 min. and to plain polyester in two hours, but that onlythe amnion/collagen coated surface exhibited complete graft coverage.

Modification of polymeric surfaces by treatment with a variety ofplasmas to accomplish various purposes is well known. The term "plasma"is used generally to describe the state of ionized gas. A plasmaconsists of high energy positively or negatively charged ions,negatively charged electrons as well as neutral species. As known in theart, a plasma may be generated by combustion, flames, physical shock ormost often by electrical discharge, such as a corona or glow discharge.In radiofrequency (RF) discharge, a substrate to be treated is placed ina vacuum chamber and gas at low pressure is bled into the system. Thegas is subjected to an RF electrical discharge, either capacitive orinductive, which generates an electromagnetic field. Ionization of thegas takes place as a result of absorption of energy from the fieldgiving high energy particles which modify the surface of the substrate.

The extent of substrate surface modification by a plasma is a functionof the number and average energy of the particles striking the surface.The energy of charged particles in a plasma is best defined by the ratio(E/p) of the electric field strength E to the background gas pressure p.This ratio is a relative measure of the average energy that an ion orelectron can gain between successive scattering collisions with neutralgas molecules. It is evident from the ratio that the energy of a plasmaparticle can be increased by either increasing the field strength ordecreasing the gas pressure. Field strength may be increased byincreasing the power of the electrical discharge; this, however, isaccompanied by additional heat formation. On the other hand, if the gaspressure is reduced too far, insufficient molecules are present forionization.

Plasmas have been used to alter surface wettability, static propertiesand receptivity of a surface to deposition of a layer of an adherentpolymeric material. Japanese Pat. No. 122529 discloses preparing asurface for graft polymerization by placing a tube in an insulatingsheath, activating an inner surface of the tube with an inductivelygenerated plasma and exposing the surface to a polymerizable monomer.

Van Wachem et al., (supra) discloses that endothelial cells can becultured on glass or glow-discharge treated polystyrene.

Garfinkle et al., in Trans. Am. Soc. Artif. Intern. Organs, 30, 432(1984) discloses plasma deposition of a fluorocarbon polymer coatingonto the luminal surface of 4-5 mm inside diameter porous polyestergrafts. In this report, an inductive plasma generated externally of thegraft penetrates to the lumen by passing through the pores of the graft.Markedly improved patency for the treated grafts is reported.

Published European Patent Application No. EP 89-124A discloses plasmatreatment of the inside of a plastic tube of 3.5 mm inside diameter byinserting the tube inside an insulating second tube and positioning theelectrodes outside of the insulating tube.

In spite of the extensive investigations on antithrombogenic prostheticdevices, the problem of thrombogenicity has not been satisfactorilysolved, in particular with respect to small diameter grafts. It istoward the solution of this problem that the current invention isdirected.

SUMMARY OF THE INVENTION

An apparatus for modifying an interior surface of an article with aplasma includes an electromagnetic field generator inside a housing. Thehousing has a connection to a vacuum source and a gas inbleed assembly.The generator is connected to an RF power source and is enclosed in adielectric in all directions except in the direction of a plasma zoneadjacent the generator. The plasma zone receives the article to beplasma treated.

In one preferred apparatus of the invention, the housing is a canisterhaving upper and lower chambers separated by a diaphragm wherein the gasinbleed is connected to the upper chamber and the vacuum connection ison the lower chamber. The preferred generator includes a plurality ofparallel plate electrodes and the dielectric is a block of highmolecular weight polyolefin having a recess in the interior whichreceives the electrodes. A bore which includes the plasma zone passesthrough the dielectric and establishes gas communication from the upperchamber through the recess and an aperture in the diaphragm to the lowerchamber. A tube to be plasma treated is positioned in the bore andextends from the upper chamber into the lower chamber. A conduit throughthe diaphragm provides gas communication between the upper and lowerchambers. The conduit controls the gas flow from the upper chamber tothe lower chamber so that a lower gas pressure may be maintained in thelower chamber. A rod passes through the top wall of the canister andengages the upper end of the tube to be plasma treated so that a longtube may be drawn slowly through the plasma zone between the electrodes.

In another preferred embodiment of the apparatus, the tube is positionedbetween support rails, and the dielectric having the electrodes disposedtherein is drawn laterally along the rails to deliver the plasma to theentire luminal wall of a long tube.

In another aspect of the invention, a method for applying a plasma to aninterior wall of a article comprises positioning the article in theplasma zone of the apparatus of the invention, evacuating the chambers,bleeding a gas into the chambers and delivering power to the electrodes.An electromagnetic field is formed which passes through the walls of thearticle and ionizes the gas inside the article to give a plasma whichtreats the interior wall of the article. In the preferred method, aluminal wall of a tube is treated by drawing the tube through the plasmazone.

An alternative method of the invention comprises holding the tubestationary between support rails and moving the dielectric having theelectrodes disposed therein along the rails from one end of the tube tothe other.

In accordance with the invention, the dielectric shields the electrodeson all sides except the facing sides so that a capacitively coupledplasma discharge is formed only in the plasma zone between theelectrodes. This arrangement allows simultaneous treatment of one ormore tubes, all of which receive an intense plasma generated at arelatively low power level. Only a low power is required because allexternal discharges which dissipate power are prevented. The low powerrequired to generate the plasma prevents heat build-up which may causethermal damage to polymeric materials of low softening points. Bycontrol of the pressure differential from one end of the tube to theother, the plasma is generated evenly in the plasma zone between theelectrodes so that the lumen surfaces of long tubes having an insidediameter (ID) as low as 2.5 mm, or even lower, are uniformly modified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a preferred apparatus for plasmageneration of the invention;

FIG. 2 is a vertical sectional view of the apparatus of FIG. 1 takenalong the line 2--2 thereof;

FIG. 3 is a horizontal sectional view of a portion of the apparatus ofFIG. 1 taken along the line 3--3 thereof;

FIG. 4 illustrates in enlarged cross-section a preferred gas flowcontrol of the apparatus of FIG. 2;

FIG. 5 is an exploded view showing the structure for opening and closingthe apparatus of FIG. 1;

FIGS. 6 and 7 are partial vertical sectional views of the apparatus ofFIG. 1 showing alternate structure for opening the apparatus;

FIG. 8a is a horizontal sectional view of the apparatus of FIG. 1 takenalong the line 8--8;

FIG. 8b is a horizontal sectional view similar to FIG. 8a, butillustrating an alternate structure for sealably opening and closing theapparatus;

FIG. 9 is a partial vertical sectional view of a modification of theapparatus of FIG. 1 taken along the line 2--2 thereof showing analternate structure for drawing the tube to be treated through theplasma zone;

FIG. 10 is a partial vertical sectional view of the apparatus of FIG. 1taken along the line 2--2 thereof showing a long tube ready for plasmatreatment;

FIG. 11 is a vertical sectional view of a modification of the apparatusof FIG. 1 taken along the line 2--2 thereof showing a simplifiedapparatus for treating a stationary tube; and

FIGS. 12 and 13 are partial vertical sectional views of the apparatus ofFIG. 1 taken along the line 2--2 thereof showing a tube in position fortreatment by an alternate embodiment of the generator-dielectric portionof the apparatus of the invention;

FIG. 14 is a perspective view of a preferred apparatus of the inventionfor generation of plasma in a long tube; and

FIG. 15 is a vertical sectional view of the apparatus of FIG. 14 takenalong the line 15--15 thereof.

DETAILED DESCRIPTION

While this invention is satisfied by embodiments in many differentforms, there will herein be described in detail preferred embodiments ofthe invention, with the understanding that the present disclosure is tobe considered as exemplary of the principles of the invention and is notintended to limit the invention to the embodiments illustrated anddescribed. The scope of the invention will be measured by the appendedclaims and their equivalents.

Ordinarily, a plama discharge generated by parallel plate electrodes isdirected throughout the entire region surrounding the electrodes. Theapparatus of the present invention prevents any substantial dischargeexcept that developed within a tube in the plasma zone between theelectrodes. The lumen wall of a tube positioned in the plasma zone issubjected to an intense plasma because none of the power applied to theelectrodes is wasted as external plasma discharge.

In accordance with the invention a glow discharge is preferred becauseit is a substantially "cold" plasma. The preferred apparatus generates aglow discharge plasma capacitively between parallel plate electrodes.The plasma produced is uniform and easily controlled and therefore givesuniform modification of the luminal wall of a small diameter tube. Aplurality of tubes may be treated at once so that the apparatus may beused for at least semi-automated tube treatment.

Referring now to the drawings, FIGS. 1 and 2 show a plasma generator 10of the invention which includes a canister 12 having an upper chamber 14having a sidewall 15 and a lower chamber 16 having a sidewall 17.Chambers 14 and 16 are separated by a diaphragm 18 having aperture 19therethrough. Upper chamber 14 has a door 20 which provides access tothe interior of the chamber, and which may be sealingly closed when theapparatus is evacuated, as described below. A top wall 21 of upperchamber 14 has an aperture 22 therethrough. A rod 23 having a handle 24for grasping projects sealingly and slidably into upper chamber 14through aperture 22.

Gas inbleed 26 includes a valve 28 and a tube 30 adapted for connectionto a gas source (not shown in the drawings). The gas source may be asingle gas or a mixture of gases mixed in conventional apparatus priorto entry into tube 30. An inlet tube 32 connects valve 28 with upperchamber 14 and passes through port 34 in top wall 21. A coaxial cable 36passes through side wall 15 and is connected to an RF power supply (notshown in the drawings). A nozzle 38 is affixed to lower chamber 16 andis adapted to be connected to a vacuum source (not shown in thedrawings). Pressure gauges 40 and 41 are connected to upper chamber 14and lower chamber 16, respectively.

As illustrated more clearly in FIG. 2, a dielectric 42 supported ondiaphragm 18 preferably consists of two blocks 43 and 44 of highmolecular weight plastic, such as polyethylene joined face to face.Recesses 45 and 46 are located in the interior of blocks 43 and 44. Forexample, recesses 45 and 46 may be machined out of blocks 43 and 44, orthe blocks may be molded to contain the recesses. Blocks 43 and 44 alsodefine grooves 47 and 48 such that, when blocks 43 and 44 are joinedface to face, recesses 45 and 46 mate to form cavity 50 and grooves 47and 48 mate to form bore 52 which passes through cavity 50.

Bore 52 receives, in a snug but sliding fit, a plastic tube 54 to beplasma treated. Tube 54 has a proximal end 55 and a distal end 56. Twoelectrodes 58 are disposed in recesses 45 and 46 in snug pressure fits.The depth of the recesses is such that when electrodes 58 are in placein the recesses, the electrodes are immediately adjacent tube 54 anddefine the plasma zone as that portion of bore 52 between theelectrodes. Coaxial cable 36 conducts power from the RF source toelectrodes 58.

Rod 23 having an internal end 59 passes through aperture 22 and extendsthrough upper chamber 14. Preferably affixed to internal end 59 of rod23 is a hook 60 adapted to be engaged with an eye 61 preferably attachedto proximal end 55 of tube 54.

A gas flow limiting conduit 64 provides gas communication throughdiaphragm 18 between upper chamber 14 and lower chamber 16, as describedin detail below.

Details of the relationship of dielectric 42, plastic tube 54 andelectrodes 58 are illustrated in FIG. 3. Electrodes 58 ae shownpositioned snugly in cavity 50 and flush with one or more tubes 54 inone or more bores 52, the electrodes being completely shielded bydielectric 42.

In plasma treatment of the luminal wall of small diameter tubes 54 inaccordance with the invention, it is preferred, though not essential,that a pressure differential be maintained between proximal end 55 anddistal end 56 of the tube. This pressure differential is preferablysmall enough to allow a uniform plasma to be generated at both ends ofthe tube yet large enough to produce a flow of the process gas throughthe tube and thereby purge away outgassed components from the tube. Ingeneral, for any given set of plasma parameters, a uniform plasma may beobtained in the plasma zone when a pressure differential of about 0 to30%, preferably about 10%, is maintained between tube ends 55 and 56.Thus, for example, if the gas pressure at proximal end 55 is 14.0 torr,the preferred pressure at distal end 56 may be about 12.6 torr.

It is evident to one skilled in the art that a pressure differential maybe developed between proximal and distal ends 55 and 56 by regulatingthe rate of gas flow through inbleed 26 and the rate of evacuationthrough nozzle 38 as monitored by pressure gauges 40 and 41 in upper andlower chambers 14 and 16 respectively. A preferred structure fordeveloping and maintaining the desired pressure differential, asillustrated in FIG. 2, is the gas flow limiting conduit 64. Conduit 64passes from upper chamber 14 to lower chamber 16 through diaphragm 18and serves to limit the gas flow between the chambers.

For some applications of the plasma generating apparatus of theinvention, the preferred pressure differential between the chambers maybe other than 10%. FIG. 4 shows a preferred means to adjust the ratiosimply by inserting a sleeve 66 inside of conduit 64. Sleeve 66 may beof any wall thickness, thereby adjusting the ratio without modificationof conduit 64 itself.

As mentioned above, the apparatus has structure to provide access to theinterior of the canister. One suitable structure is illustrated in FIGS.1 and 5 as door 20. FIG. 5 shows door 20 mating with an opening 70 inside wall 15 of upper chamber 14. Door 20 preferably has pegs 72 at thecorners which enter slots 74 of side wall 15, the pegs thereby servingto locate door 20 over opening 70. An O-ring 76 in a groove 78 of sidewall 15 forms a seal with door 20 when vacuum is applied through nozzle38.

FIGS. 6-8 show structures, alternate to the door of FIG. 5, for openingcanister 12. (In the following discussion of alternate embodiments ofthe invention, elements which correspond to elements previouslydescribed with respect to the apparatus of FIG. 1 are given the samebase number followed by a lower case letter.)

In FIG. 6, bottom wall 81 of lower chamber 16a has an opening 82 and athread 83 on the inner surface of wall 81. A groove 84 in bottom wall 81receives an O-ring 85. Cover plate 86 has a thread 87 which mates withthread 83 whereby upper surface 88 of cover plate 86 sealingly engagesO-ring 85.

As shown in FIG. 7, chambers 14b and 16b are removably affixed andsealed by mating threads 90 and 92 on side walls 15b and 17brespectively. Any conventional means for sealing the thread joint, suchas grease, may be used.

The upper and lower chambers may be separable. FIG. 8a shows side wall17c of lower chamber 16c having a flat upper surface 104. FIG. 8billustrates an O-ring 100 in a groove 102 of flat upper surface 104.O-ring 100 sealingly engages the lower surface of the sidewall of theupper chamber when vacuum is applied through nozzle 38.

Structures (not shown in the drawings) other than the hook 60 and eye 61illustrated in FIG. 2 may be used to affix rod 23 to tube 54. Forexample, internal end 59 of rod 23 and proximal end 55 of tube 54 may beaffixed by a clamp or merely be tied together with cord or wire.Alternatively, a small magnet inserted by a pressure fit into proximalend 55 of the tube 54 mnay engage a piece of magnetic material attachedto internal end 59 of rod 23.

An alternate embodiment of the invention, illustrated in FIG. 9, alsouses magnetism to draw the tube through the plasma zone and at the sametime eliminates the sliding seal between rod 23 and top wall 21 of FIG.2, which may be a source of leakage. In FIG. 9, a preferably slenderglass casing 110 having closed end 111 is permanently sealed intoaperture 22d of top wall 21d. Rod 113, preferably of a magneticmaterial, or glass having magnetic bands thereon, or other magneticmaterial, is disposed slidably in casing 110. End 114 of rod 113 isaffixed by any suitable means as described above, to proximal end 55d oftube 54d. Magnet 115, when placed on outside wall 116 of casing 110 maybe slid upwardly to cause rod 113 to slide in casing 110 and draw tube54d with it.

Any length of tube 54 may be treated with the apparatus and method ofthe invention. It is evident from FIG. 2 that distal end 56 may be theend of a coil of tube 54 disposed in lower chamber 16. FIG. 10illustrates coil 120 of tube 54e having distal end 56e. Preferably coil120 has a plurality of holes 122 spaced about 1 meter apart to aid inpassage of gas through tube 54e. In this embodiment of the invention, ithas been found that holes 122 preferably have a diameter substantiallythe same as the ID of tube 54e. When plasma treating coil 120, it isconvenient to use the embodiment of the canister illustrated in FIG. 6having access to the lower chamber for insertion of the coil.

For some plasma treatments, a simplified apparatus, as shown in FIG. 11may be suitable. It is seen that FIG. 11 is similar to the apparatus ofFIGS. 1 and 2 except it lacks diaphragm 18, conduit 64, gauges 40 and 41and the structure by which the tube is drawn through the plasma zone. InFIG. 11 plasma generator 10f includes a canister 12f having dielectric42f supported therein on circumferential rim 130. Dielectric 42fconsists of blocks 43f and 44f having recesses 45f and 46f which definecavity 50f. Grooves 47f and 48f mate to form bore 52f which receivestube 54f. Electrodes 58f are positioned in recesses 45f and 46f anddefine a plasma zone in bore 52f therebetween.

FIG. 12 shows a plurality of ring electrodes 134 around tube 54g andwithin dielectric 42g. Leads 136 connect ring electrodes 134 to a powersource (not shown). Electrodes 134 may be spaced about 1-10, preferablyabout 2-5 cm apart.

Although plasma generated capacitively between parallel plate electrodesis preferred, an inductively generated plasma may alsio be used to treata luminal surface in accordance with the invention. FIG. 13 illustratesan arrangement suitable for this embodiment of the invention. Coil 140having leads 142 connected to a power source (not shown) is wrappedaround tube 54h and is completely encased in dielectric 42h.

Still other arrangements of the elements of the generator of theinvention for plasma treatment of tube lumens with electrodes shieldedby a dielectric may be envisioned. For example, a stationary tube may bepositioned adjacent an electrode encased in a dielectric, and theelectrode-dielectric unit moved past the tube to generate the plasma inthe tube lumen.

FIGS. 14 and 15 illustrate an embodiment of the plasma generator of theinvention including a moving electrode-dielectric assembly which isparticularly suitable for plasma treatment of the lumen walls of longtubes. The embodiment shown in these figures is preferably disposedsubstantially horizontally.

A plasma generator 200 includes a housing 202, preferably cylindrical,having a proximal end plate 204, a distal end plate 206 and a side wall208. Although housing 202 can be of any suitable material, such asmetal, ceramic, plastic or glass, it will be illustrated in FIG. 14 forthe preferred glass or transparent plastic so that the relationship ofthe internal elements may be readily visualized.

Housing 202 is divided into proximal chamber 210 and distal chamber 212by a diaphragm 214 having a hole 216 therethrough for receiving a tube218. A gas flow limiting conduit 220 and optional sleeves provide gascommunication through diaphragm 214 as described above for conduit 64.

End plate 204 and diaphragm 214 are sealingly engaged to sidewall 208 byany suitable means, preferably by an O-ring (not shown). End plate 206may also be sealingly engaged to sidewall 208, but preferably isintegral with the sidewall.

Gas inbleed 222 and pressure gauge 223 pass through distal end plate206. Vacuum nozzle 224 and pressure gauge 225 pass through proximal endplate 204, all forming vacuum tight seals with their respective endplates.

Removably positioned within distal chamber 212 is anelectrode-dielectric assembly 230 including electrodes 231, dielectric232, upper tube support rail 234, lower tube support rail 236, distalclamp 238 and proximal clamp 240. Coaxial cable 242 passes sealinglythrough hole 244 in distal end plate 206 and delivers RF power to theelectrodes. Magnet 246 is secured to dielectric 232 by any suitablemeans, as for example glue.

As shown in FIG. 15, electrodes 231 fit snugly in cavities 248 indielectric 232, as described above for electrodes 58. Tubing 218 ispositioned between upper rail 234 and lower rail 236 in the plasma zoneimmediately adjacent electrodes 231. Assembly 230 is adapted forwithdrawal from housing 202 for insertion of tube 218 as describedbelow.

All embodiments of the apparatus of the invention as heretoforedescribed may be used with a conventional high frequency RF generatorand impedance matching network and a conventional vacuum system. Suchequipment is well known in the art (as, for example, in U.S. Pat. No.3,847,652) and further details with respect to these aspects of theinvention are not needed for a complete understanding of the invention.

In preparation of generator 10 for use, canister 12 is opened and tube54 to be plasma-treated is inserted into bore 52 so that it occupies theplasma zone between electrodes 58. Eye 61 is attached to proximal end 55of the tube by any suitable means. Hook 60 on internal end 59 of rod 23is engaged with eye 61, and the canister 10 is sealingly closed.

Inserting a tube into generator 200 may be carried out by removingproximal end plate 204 and diaphragm 214 from housing 202 and slidingassembly 230 forward until completely removed from housing 202. Clamps238 and 240 are opened and removed, and dielectric 232 is slid overupper and lower rails 234 and 236 until disengaged therefrom. The railsare then separated and a tube 218 to be plasma treated is placedtherebetween. The generator is then reassembled by reversing thesesteps.

For plasma treatment of the tube, the loaded and assembled generator 10or 200 is evacuated by attaching the vacuum nozzle to a vacuum pump. Gasfrom a gas source is bled into the evacuated apparatus through the gasinbleed until the desired gas pressure differential across the conduitis obtained. As mentioned above, one or more sleeves may be insertedinto the conduit if it is desired to reduce the diameter of the conduit.An RF electromagnetic field is generated in the plasma zone by applyingcurrent of the desired frequency to the electrodes from the RFgenerator. Ionization of the gas in the tube is induced by the field,and the resulting plasma in the tube modifies the luminal wall of thesection of tube in the plasma zone.

If it is contemplated to plasma-treat a length of tube equivalent to orless than the length of the electrodes, an apparatus in accordance withFIG. 11 may preferably be used. If the length of tube to be treated isgreater than the length of the electrodes, the entire tube may betreated with the apparatus of FIG. 10 or, preferably, with the apparatusof FIG. 14. An external magnet (not shown in FIG. 14) is placed directlyabove magnet 246 on the outside of side wall 208. The two magnets arethereby magnetically engaged so that lateral movement of the externalmagnet along the side wall causes the dielectric-electrode unit to slidealong the rails in either direction, as shown by the dotted arrows inFIG. 14. Determination of a suitable rate for drawing the tube of FIG.10 or the electrode-dielectric unit of FIG. 14 to give the desireddegree of surface modification is well within the purview of one skilledin the art.

The apparatus and method of the invention may be used to treat a luminalsurface with a plasma generated from any gas under any suitable plasmaparameters to be determined in accordance with the desired surfacetreatment. Thus, without wishing to be limited thereby, the gas may beammonia, nitrogen, neon, argon, xenon, krypton, oxygen or mixturesthereof. In addition, the gas may be a vaporized organic material, suchas an ethylenic monomer or a lower molecular weight siloxane to beplasma polymerized or deposited on the luminal wall of the tube.

Suitable plasma parameters may be power levels from about 10 to 1000watts, RF frequency of about 1 to 100 megaherz, exposure times of about5 seconds to 12 hours, gas pressures of about 0.1 to 100 torr and a gasflow rate of about 1-200 standard cc/sec.

In accordance with the method of the invention in which a small diameterconduit is plasma treated to modify the luminal wall in preparation forattachment of endothelial cells, a preferred plasma is generated usingthe apparatus of the invention from ammonia or nitrogen with a powerlevel of 50-125 watts, an RF frequency of about 8-30 megaherz, anexposure time for a particular area of the tube of about 0.2 to 2.0min., a gas pressure of about 1-20 torr and gas flow rate of about 5 to20 standard cc/sec.

Dielectrics 42 and 232 may be of any material which prevents theelectromagnetic field from being applied in any direction other thaninto the plasma zone between the electrodes. Suitable materials are, forexample, glass, rubber, ceramic and, preferably, a high molecular weightpolyolefin such as polypropylene or polyethylene. It has been found thatwhen the electrodes are encased with about 1 to 5, preferably about 21/2cm of dielectric material, sufficient shielding is provided so thatsubstantially no plasma is formed external of the plasma zone.

Suitable electrodes may be of any conducting material, although aluminumand stainless steel are preferred. Preferred electrodes are from 2 to 10cm long although any length consistent with the dimensions of thehousing are suitable. Likewise, the width and height of the electrodesare not critical, although preferred electrodes are about 0.1 cm inthickness and about 0.5 to 2.0 cm in width.

As mentioned above, housing 202 may be metal, plastic or, preferably,glass. It is of course understood by one skilled in the art that a metalhousing must be nonferrous when magnets are to be used to move theelectrodes. The tube support rails may likewise be glass or plastic,preferably a plastic having a low friction surface.

While the apparatus of the invention has been described in detail forthe plasma-treatment of the lumen of a small diameter tube, it isapparent that merely by altering the dimensions of the dielectric andthe electrodes, any article having an internal surface which can becontacted with a plasma gas can be treated. The article to beplasma-treated may be of any non-conducting material such as glass,plastic, ceramic, rubber and composites thereof. Conducting materialssuch as metals cannot be treated on internal surfaces with the apparatusof the invention because electromagnetic fields do not pass throughconductors. A preferred material for a vascular graft is polyurethanebecause its high degree of compliance and flexibility makes it mostsimilar to a human blood vessel.

Thus, the apparatus of the invention generates a plasma in the lumen ofa tube as small as 2.5 mm ID, or even smaller. The plasma is generatedin a plasma zone between electrodes shielded by a dielectric whichprevents substantially all plasma generation external to the plasmazone. By limiting plasma generation to the plasma zone, no power iswasted so that the desired plasma is generated inside of the tube in theplasma zone without application of excessive power to the electrodes. Asa result, heat buildup is minimized allowing plasma treatment of theluminal wall of a tube made of a heat sensitive material.

What is claimed is:
 1. An apparatus for applying a plasma to an interiorwall of a plastic article comprising:(a) a housing having first andsecond chambers separated by a diaphragm defining an aperture forreceiving an article to be plasma treated; (b) a dielectric within saidfirst chamber having a bore therethrough, said bore including a cavityin the interior of said dielectric and a plasma zone for receiving saidarticle; (c) an electrode positioned in said cavity and surrounded bysaid dielectric in all directions except toward said plasma zone anddefining said plasma zone; (d) means for opening and sealably closingsaid housing; (e) means for delivering power to said electrode; (f)means for conducting a gas to said first chamber; and (g) means forconnecting said second chamber to a vacuum source.
 2. The apparatus ofclaim 1 wherein said means for delivering is a coaxial cable.
 3. Theapparatus of claim 1 wherein said means for conducting includes a valvefor controlling the flow of said gas.
 4. The apparatus of claim 1wherein said means for connecting is a nozzle.
 5. The apparatus of claim1 further comprising means for determining the pressure in said firstand second chambers.
 6. The apparatus of claim 1 further comprisingmeans for maintaining a pressure differential between said first andsecond chambers.
 7. The apparatus of claim 6 wherein said means formaintaining is a conduit through said diaphragm.
 8. The apparatus ofclaim 1 further comprising drawing means passing sealably through a topwall of said housing for drawing said article through said plasma zone.9. The apparatus of claim 8 wherein said drawing means comprises a rodpassing slidably through said top wall, said rod having an internal endinside of said upper chamber and an external end outside of said upperchamber, said internal end having affixing means thereon.
 10. Theapparatus of claim 9 wherein said affixing means is a hook.
 11. Theapparatus of claim 9 wherein said affixing means is a magnetic material.12. The apparatus of claim 8 wherein said drawing means is a casinghaving a closed end external of said canister, said casing havingmagnetic means slidably disposed therein, said magnetic means havingaffixing means on an end thereof inside of said canister.
 13. Theapparatus of claim 1 wherein said housing comprises separable top andbottom portions sealably engaged by mating threads.
 14. The apparatus ofclaim 1 wherein said housing comprises separable top and bottom portionssealably engaged by an O-ring.
 15. The apparatus in accordance withclaim 1 wherein said dielectric is fabricated from a material selectedfrom the group consisting of glass, plastic, rubber and ceramic.
 16. Theapparatus in accordance with claim 15 wherein said plastic is apolyolefin.
 17. The apparatus in accordance with claim 1 furthercomprising a support rail for said article passing through said bore,said dielectric being slidably mounted on said rail.
 18. The apparatusin accordance with claim 17 further comprising a clamp for positioningsaid rail in said housing.
 19. An apparatus for applying a plasma to aninterior wall of a non-conductive article comprising:(a) a housingenclosing a chamber; (b) shielding means within said chamber having abore therethrough, said bore including a cavity and a plasma zone forreceiving an article to be plasma treated; (c) generating means for anelectromagnetic field in said cavity and surrounded by said shieldingmeans in all directions except toward said plasma zone; (d) means foropening and sealably closing said housing; (e) means for deliveringpower to said generating means; (f) means for conducting a gas to saidchamber; and (g) means for connecting said chamber to a vacuum source.20. The apparatus in accordance with claim 19 wherein said generatingmeans is an electrode adapted to generate a capacitively coupleddischarge plasma.
 21. The apparatus in accordance with claim 19 whereinsaid generating means is a coil adapted to generate an inductivelycoupled discharge plasma.
 22. An apparatus for applying a plasma to theluminal wall of a plastic tube comprising:(a) a canister having a sidewall, a top wall and a diaphragm, said diaphragm dividing said canisterinto an upper chamber and a lower chamber and defining an aperture; (b)a dielectric within said upper chamber, said dielectric having a boretherethrough, said bore communicating through said aperture with saidupper and lower chambers, said bore including a cavity in the interiorof said dielectric and a plasma zone for receiving a tube to be plasmatreated; (c) a plurality of electrodes positioned in said cavity andsurrounded by said dielectric in all directions except toward saidplasma zone and defining said plasma zone therebetween; (d) means foropening and sealably closing said canister; (e) a conduit through saiddiaphragm for maintaining a pressure differential between said upper andlower chambers; (f) drawing means passing sealably through said topwall; (g) means for delivering electrical power to said electrodes; (h)an inbleed assembly for conducting gas into said upper chamber; and (i)a nozzle for connecting said lower chamber to a vacuum source.
 23. Theapparatus in accordance with claim 22 wherein said drawing meanscomprises a rod having an internal end inside of said upper chamber andan external end outside of said upper chamber, said internal end havingaffixing means thereon.
 24. The apparatus in accordance with claim 22wherein said drawing means is a tube having a closed end external ofsaid canister, said tube having magnetic means slidably disposedtherein, said magnetic means having affixing means on an end thereofinside of said canister.
 25. An apparatus for applying a plasma to theluminal wall of a plastic tube comprising:(a) a housing having a firstchamber and a second chamber separated by a diaphragm defining anaperture for receiving a tube to be plasma treated, a first end plateremovably affixed to said housing defining sid first chamber and asecond end plate affixed to said housing and defining said secondchamber; (b) a dielectric within said first chamber, said dielectrichaving a bore therethrough, said bore including a cavity in the interiorof said dielectric and a plasma zone for receiving said tube; (c) aplurality of electrodes positioned in said cavity and surrounded by saiddielectric in all directions except toward said plasma zone and definingsaid plasma zone therebetween; (d) mating upper and lower support railsfor said tube within said first chamber and passing through said bore,said dielectric being slidably mounted on said rails; (e) means forremovably mating said upper and lower rails; (f) a conduit through saiddiaphragm for maintaining a pressure differential between said first andsecond chambers; (g) means for moving said dielectric on said rails; (h)means for delivering electrical power to said electrodes; (i) an inbleedassembly for conducting gas into said first chamber; and (j) a nozzlefor connecting said second chamber to a vacuum source.
 26. The apparatusin accordance with claim 25 wherein said means for mating is a clamp.27. The apparatus in accordance with claim 25 wherein said means formoving said dielectric is a first magnet affixed to said dielectric andadapted for magnetic engagement with a second magnet slidably positionedon said housing.